It should already work.
Administrators can create users even when registrations are closed. And I developed this feature for this case: a closed instance, where admins can invite users.
@JohnLivingston are you talking about the invitation link to create a password ? in this case the admin creates the account (with a username filled by the admin and an automatic channel name) and invite the user to create a password.
I’m talking about a fearure to send invitation link to create the whole account, a unique link with a hash redirecting to the signup view allowing the user to choose its username and channel name.